Yu Yan
Calcium silicate hydrate (C-S-H) is the main hydration product in Portland and blended cements, and greatly affects durability and mechanical properties of the hydrated cement. In the presence of Al-rich supplementary cementitious materials (SCMs), C-(A-)S-H can contain more Al than C-S-H in plain Portland cements. The use of SCMs in cementitious system not only lowers CO2 emission, but may also enhance the mechanical properties in the long term, its durability and can help to suppress alkali silica reaction. However, important questions regarding the effect of Al, alkali and equilibration time on C-A-S-H structure and solubility are still not fully understood. This thesis focuses on the interplay between aluminium, alkali hydroxides in solution, the structure of C-A-S-H and aluminium, alkali uptake by C-A-S-H and kinetics of C-A-S-H formation. For aluminium free C-S-H, KOH and NaOH have a similar effect, they increase pH values and silicon concentrations, and decrease calcium concentrations. At higher alkali hydroxide concentrations, more portlandite precipitates, while amorphous silica dissolves. This increases the Ca/SiC-S-H at low Ca/Sitarget but lowers the maximum Ca/SiC-S-H from 1.5 to 1.2 in 1 M KOH/NaOH. The amount of alkalis bound in C-S-H increases with increasing alkali hydroxide concentrations and is higher at low Ca/SiC-S-H. KOH/NaOH lead to a structural rearrangement in C-S-H, increasing the interlayer distance, number of layers stacked in c direction and shortening the silica chains. Comparison with the independently developed CASH+ thermodynamic model showed a good agreement between the observed and modelled changes, including the shortening of the MCL.For C-A-S-H with Ca/Si=1, the presence of Al led to higher amounts of secondary phases, larger interlayer distance, longer silicate chain, higher dissolved Al and Si, and lower Ca concentrations. The amount of secondary phases is reduced at higher pH; strätlingite and Al(OH)3 dominate at lower pH, and katoite at higher pH values. High pH is also associated with shortened silicate chain length, increased Al and Si concentrations, maximum Al/SiC-S-H and lower Ca concentrations. The Al uptake in C-A-S-H increased with aluminum concentration in solution, indicating the predominance of a common Al sorption mechanism independent of dissolved Al concentration or pH values. XANES data showed the presence of both Al(IV) and Al(VI) with different chemical environments in C-S-H.The effect of equilibration time on C-A-S-H was investigated from 6 hours to 3 years. Rapid changes within the first day were observed in both the solid phase and the aqueous phase. Portlandite was formed in all samples with different Ca/Si and its amount decreased rapidly within the first day and remained stable after more than 7 days. Gismondine-P1 precipitated between 1 and 3 years and destabilized the C-A-S-H phases at target Ca/Si 0.6. In the solid phase, the Al migration from secondary phases to C-A-S-H and an increasing fraction of Al(V) and Al(VI) in C-S-H were observed with longer equilibration time. The Ca/Si ratio of C-S-H played an important role on the extent of changes in aqueous concentration: an increasing of Al concentrations with time was observed in C-A-S-H with Ca/Sitarget=0.6, while a decrease was observed for C-A-S-H with Ca/Sitarget=1.0. The uptake of Al and Na in C-A-S-H was observed to decrease after 4 days and 1 day separately, which indicates a rearrangement of C-A-S-H with time.
EPFL2022
